COMPARATIVE FITNESS OF 3 STRAINS OF COLORADO POTATO BEETLE (COLEOPTERA, CHRYSOMELIDAE) IN THE FIELD - SPATIAL AND TEMPORAL VARIATION IN INSECTICIDE SELECTION

Fitness components were measured for permethrin-resistant, susceptible , and hybrid strains of Colorado potato beetle, Leptinotarsa decemline ata (Say), on insecticide-treated and untreated potato, Solanum tubero sum L., under field conditions. The hybrid strain was created by cross ing resistant females and susceptible males. In separate experiments, survivorship was determined for adults and larvae of various ages (1, 2, 4, and 6 d) that were exposed directly to a permethrin (Ambush 2EC) spray. Results indicated that there is significant selection against susceptible genotypes during aIl larval stages and the adult stage. On e- and 2-d-old larvae, as well as adults, of the hybrid strain and the resistant strain had similar survival. Survival in 4- and 6-d-old lar vae of the hybrid strain was intermediate between the susceptible and resistant strains. Weights of 4- and 6-d-old larvae of the resistant a nd hybrid strains that survived treatment with insecticide were signif icantly lower than weights of untreated larvae. In another set of expe riments, viability of 1-d-old larvae and viability and fecundity of ad ults were estimated when beetles were placed on potato foliage at vari ous intervals (2, 5, 8 and 13 d) after permethrin application. Selecti on against susceptible 1-d-old larvae was strongest when placement was 2 d after application and selection diminished on subsequent placemen t dates but remained significant for the duration of the experiment. S election against susceptible larvae was more intense on old foliage th an new foliage. Larvae of the hybrid and resistant strains had similar survival on treated foliage. When hybrid 1-d-old larvae were placed o n foliage 2 and 5 d after application, the sex ratios of survivors wer e 91 and 75% male, respectively. These values were significantly highe r than that of the resistant strain survivors on treated plants and hy brid survivors on untreated plants. This is consistent with the report ed sex-linkage of this trait in Colorado potato beetle, but surprising ly, survivorship of hybrid males was significantly higher than survivo rship in the resistant strain males for day 2 and 5 placements. Differ ences were not detected in survivorship and fecundity of adults ofthe three strains placed on plants at various intervals after insecticide application. Implications of these fitness studies for resistance mana gement of Colorado potato beetle are discussed.